The man who has run Moscow like his personal fiefdom for the past 18 years, leaving a greater imprint on Russia's capital than any other leader since Stalin, is in imminent danger of losing his job, Kremlin insiders said today.

Yuri Luzhkov, the plain-speaking mayor of Moscow, is fighting for his political life following a string of damaging corruption allegations and a campaign by shadowy forces inside the Kremlin to unseat him.

At stake is control over Moscow city council's $35bn (£24bn) budget, lucrative building and development tenders, and the highest profile political platform in Russia behind the presidency and prime minister's office.

There is also Luzhkov's personal fortune. Inevitably, it is linked to that of his wife, property developer Yelena Baturina, Russia's only woman billionaire. (Her post-crisis assets are put at $1bn, down from $4.2bn in 2008.)

Luzhkov is post-Soviet Russia's great survivor, fighting off numerous attempts to unseat him. His enemies now believe he is finally on his way out – with the Kremlin plotting to replace him ahead of parliamentary elections next year and Vladimir Putin's likely return as president in 2012.

"He's not as strong as he once was. He's grown weaker," Nikolai Petrov, a Kremlin expert at Moscow's Carnegie Centre, said today. "The chance of him being replaced in two to three months is high."

Against the protean backdrop of Russia's politics Luzhkov has been an immovable point. Since becoming mayor in 1992 he has seen three changes of president, six parliaments and 10 prime ministers – not to mention two Chechen wars and a major financial crisis. This week he clocked up 18 years in power.

Supporters say he has transformed Moscow from a crumbling communist shell into a vibrant modern metropolis. He has built an inner and an outer city ring road, paid pensions promptly, ensured the hot water works, and even rebuilt Christ the Saviour's, the gold-domed orthodox cathedral knocked down by Stalin.

Critics dub him a Soviet-era throwback. "He's a populist kolkhoznik [collective farm worker]," said Nikolai Alekseev, an activist angry at Luzhkov's ban on gay parades. The European court of human rights is expected to overturn the ban.

Ultimately, however, it is allegations of sleaze that have made Luzhkov's mayoral position untenable. Municipal corruption is not unusual in Russia: indeed, according to opposition leader Vladimir Ryzhkov, it is an essential part of Putin's "power vertical". But since 2000, when Putin's arrival in the Kremlin dashed Luzhkov's presidential ambitions, corruption in the capital has flourished, Ryzhkov says.

Baturina vehemently rejects claims her husband's job has helped her amass a fortune. But last year a former business partner, Chalva Tchigirinski, lifted the lid on Moscow's opaque property world. In a deposition to the high court in London he alleged: "Ms Baturina's sphere of influence is such that no major project can succeed [in Moscow] without her backing."

Tchigirinski said he paid bills for the maintenance of Baturina's private jet, and said she hid sensitive documents in a Swiss deposit box. She has issued a blanket denial of all Tchigirinski's claims, calling them "the exact opposite of reality".

Last year, the opposition leader, Boris Nemtsov, published a lengthy dossier criticising Luzhkov's tenure. The mayor sued. But unusually in a city under his sway, where judges have always ruled in his favour, he secured the retraction of only one out of six statements.

Nemtsov told the Guardian he was "100% convinced" Luzhkov would be fired. "There are two Luzhkovs. The first in the 90s was quite progressive and supported small business. The second is terrible. Traffic is disastrous, environment is neglected, and real estate prices are among the world's highest because of monopolisation and corruption."

Detractors also accuse Luzhkov of bulldozing Moscow's architectural heritage, and replacing it with mock-palaces. This week the mayor defended developers who levelled historic buildings in the Kadashi district next to a baroque church. They are building a housing complex in its place.

"Luzhkov represents the interests of building companies," protester Sergei Akeev said, speaking on the picket line.

Luzhkov's term expires in June next year, when he will be almost 75. Some experts believe the president, Dmitry Medvedev, will refuse to reappoint him. Putin acidly commented recently that Luzhkov was keeping the "juiciest parcels" of Moscow land for himself.

"Medvedev looks silly when he talks about fighting corruption when he has Mr Luzhkov right next to him as mayor of Moscow," Vladimir Kara-Murza, a Washington-based analyst, said. "It makes Medvedev look powerless."

But at a time of mounting social protest the Kremlin may decide to play it safe. Last month Luzhkov sent in riot police to break up an opposition rally in Triumphal Square. More than 150 activists were arrested. "I've heard rumours of Luzhkov's demise for the past 18 years, almost every day. But he's still in his position," Ryzhkov pointed out.

A former state chemical industry bureaucrat, Luzhkov rarely deals with foreigners, and declined to comment.